Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of an amount of gas is so much greater than the volume of the same number of gas molecules in a liquefied state, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas and is not economical.
Formations that include small amounts of natural gas may include primarily oil, with the natural gas being a byproduct of oil production that is thus termed associated gas. In the past, associated gas has typically been flared, i.e., burned in the ambient air. However, current environmental concerns and regulations discourage or prohibit this practice.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline, jet fuel, kerosene, and diesel fuel have been decreasing and supplies are not expected to meet demand in the coming years. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures.
The above is also known as GTL (gas to liquid), i.e. the conversion of natural gas into other, heavier hydrocarbons. Instead of using gas, also coal or biomass can be used as the feed stock for making synthesis gas. For the coal feedstock, the process is known as CTL, when based on biomass the abbreviation BTL is commonly used. The general principle is to convert the feedstock into synthesis gas which is then converted into the desired hydrocarbons. The term XTL is also sometimes used to describe the general process, X stands in this case for any feedstock that can be converted into synthesis gas.
One method for converting natural gas, coal and/or biomass to liquid fuels involves two sequential chemical transformations. In case of GTL, the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen and/or steam to form syngas, which is a combination of predominantly carbon monoxide gas and hydrogen gas. Syngas (or synthesis gas) can also contain carbon dioxide. In case of CTL, coal is gasified to syngas. In case of BTL, biomass is gasified to syngas. The production of syngas itself can normally includes multiple steps. After making the CO/H2 containing gas mixture, the syngas often needs to be purified to remove certain substances that would cause problems downstream in the Fischer-Tropsch section. After the syngas has been optionally purified, the second transformation, known as the Fischer-Tropsch process takes place. The predominant reaction is between carbon monoxide and hydrogen to form organic molecules containing carbon and hydrogen. Those molecules containing only carbon and hydrogen are known as hydrocarbons. Those molecules containing oxygen in addition to carbon and hydrogen are known as oxygenates. Hydrocarbons having carbons linked in a straight chain are known as aliphatics and are particularly desirable as the basis of synthetic diesel fuel.
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification). The molecules react to form hydrocarbons while confined on the surface of the catalyst. The hydrocarbon products then desorb from the catalyst and can be collected. H. Schulz (Applied Catalysis A: General 1999, 186, p 3) gives an overview of trends in Fischer-Tropsch catalysis.
The catalyst may be contacted with synthesis gas in a variety of reaction zones that may include one or more reactors. Common reactors include packed bed (also termed fixed bed) reactors, slurry bed reactors, and fluidized bed reactors. Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as poor temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors sometimes called “slurry reactors” or “slurry bubble columns,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid by using different techniques like filtration, settling, hydrocyclones, magnetic techniques, etc. Gas-agitated multiphase reactors or slurry bubble column reactors (SBCRs) inherently have very high heat transfer rates; therefore, reduced reactor cost and the ability to remove and add catalyst online are principal advantages of such reactors in Fischer-Tropsch synthesis, which is exothermic. Sie and Krishna (Applied Catalysis A: General 1999, 186, p. 55) give a history of the development of various Fischer Tropsch reactors.
Typically the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus having a range of molecular weights. Thus, the Fischer-Tropsch products produced by conversion of synthesis gas commonly contains a range of hydrocarbons including gases, liquids and waxes. It is highly desirable to maximize the production of high-value liquid hydrocarbons, such as hydrocarbons with at least 5 carbon atoms per hydrocarbon chain (C.sub.5+ hydrocarbons).
The composition of a catalyst influences the relative amounts of hydrocarbons obtained from a Fischer-Tropsch catalytic process. Cobalt metal is particularly desirable in catalysts used in converting synthesis gas to hydrocarbons suitable for the production of diesel fuel. Further, iron, nickel, and ruthenium have been used in Fischer-Tropsch catalysts. Nickel catalysts favor termination of the chain growth and are useful for aiding the selective production of methane from syngas. Iron has the advantage of being readily available and relatively inexpensive but the disadvantage of a water-gas shift activity. Ruthenium has the advantage of high activity but is quite expensive.
One of the limitations of a Fischer-Tropsch process is that the activity of the catalyst will, due to a number of factors, deteriorate over time.
The commercial incentives for a process to convert synthesis gas to liquid fuels and other products are increasing as the need for energy sources increases. One successful approach to meeting this need has been to make synthesis gas and then synthetically convert the synthesis gas into heavier hydrocarbons (C5+) through the Fischer-Tropsch (F-T) process. The synthetic production of hydrocarbons by the catalytic reaction of synthesis gas is well known and is generally referred to as the Fischer-Tropsch reaction. This F-T process was developed approximately eighty years ago in Germany, and since then, it has been practiced commercially in Germany during World War II and later in South Africa. In recent times, very large, new GTL and CTL complexes are built in other countries as well.
Fischer-Tropsch hydrocarbon conversion systems typically have a synthesis gas generator and a Fischer-Tropsch reactor unit. In the case of starting with a gas feed stock, the synthesis gas generator receives light, short-chain hydrocarbons such as methane and produces synthesis gas. The synthesis gas is then delivered to a Fischer-Tropsch reactor. In the F-T reactor, the synthesis gas is primarily converted to useful C5+ hydrocarbons. Recent examples of Fischer-Tropsch systems are included in U.S. Pat. Nos. 4,973,453; 5,733,941; and 5,861,441.
Numerous types of reactor systems have been used for carrying out the Fischer-Tropsch reaction. See generally the many examples found on www.fischertropsch.org. The commercial development of the Fischer-Tropsch reactor systems has included conventional fixed-bed and three-phase slurry bubble column designs or other moving-bed designs. But, due to the complicated interplay between heat and mass transfer and the relatively high cost of Fischer-Tropsch catalysts, no single reactor design has dominated the commercial developments to date.
Fischer-Tropsch three-phase bubble column reactors or the like appear to offer distinct advantages over the fixed-bed design in terms of heat transfer and diffusion characteristics. One particular type of three-phase bubble column is the slurry bubble column, wherein the catalyst size is generally between 10 and 200 microns (μm). Three-phase bubble column reactors present a number of technical challenges.
The technical challenges associated with three-phase bubble columns include solids management. One particular challenge in this area is to efficiently rejuvenate slurry catalysts. When a slurry Fischer-Tropsch catalyst is used over time, it has a disadvantage of slowly, but reversibly, deactivating compared to its initial catalytic activity. As the synthesis gas (primarily H2 and CO) is fed to the Fischer-Tropsch reactor and converted with the F-T catalyst, the catalyst experiences deactivation caused by carbon build up, physical degradation, and the effects of trace compounds other than CO and H2, such as by nitrogen containing species or oxygenated byproducts. “Carbon build up” references the accumulation of heavy hydrocarbons and carbonaceous type material that can have a hydrogen content less than that of F-T products. To remedy the deactivation, the catalyst is regenerated, or rejuvenated, using any of a number of techniques.
Rejuvenation is different from the initial activation of the Fischer-Tropsch catalyst. For cobalt catalysts, the initial activation involves converting the cobalt to a reduced state. An example of an initial activation technique is found U.S. Pat. No. 4,729,981, entitled “ROR-Activated Catalyst for Synthesis Gas Conversion,” which describes the initial preparation of a cobalt or nickel based Fischer-Tropsch catalyst by reducing it in hydrogen, oxidizing it in an oxygen-containing gas, and then reducing it in hydrogen. The catalyst is then ready for its initial use. Once in use, it will begin to deactivate, and it will need regeneration.
Regeneration of a Fischer-Tropsch catalyst after activation and operation has long been known to restore the activity of the catalyst. See, e.g., H. H. Storch et al., The Fischer-Tropsch And Related Synthesis (Wiley: New York 1951), 211 222. Storch describes using hydrogen treatments to restore the catalyst activity. There are many other examples. For example, U.S. Pat. No. 2,159,140 describes pulling the catalyst from the reactor (where it appears to have been fluidized) and removing the catalyst and treating it with hydrogen to regenerate the catalyst. U.S. Pat. No. 2,238,726 indicates that the non-volatile reaction products can be removed from the catalyst by treating it with hydrogen or gases or vapors containing hydrogen and that this can be done in the midst of oil circulation. Col. 2:34 54. As another example, U.S. Pat. No. 2,616,911 describes oxidizing the catalyst and then reducing it while maintaining it in suspension or a fluidized state. Other examples relating to regenerating and/or de-waxing Fischer-Tropsch catalysts include U.S. Pat. Nos. 6,323,248 B1; 6,201,030 B1; 5,844,005; 5,292,705; 2,247,087; 2,259,961; 2,289,731; 2,458,870; 2,518,337; and 2,440,109.
Regenerating a slurry catalyst presents particular challenges, because the catalyst is in slurry form. Elaborate efforts have been made to separate the catalyst to allow regeneration outside the Fischer-Tropsch reactor or to regenerate it in-situ. The rejuvenation can be carried out intermittently or continuously.
As an example of a regeneration process, U.S. Pat. No. 5,973,012 describes a reversibly deactivated, particulate slurry catalyst that is rejuvenated by circulating the slurry from a slurry body through (i) a gas disengaging zone to remove gas bubbles from the slurry, (ii) a catalyst rejuvenation zone in which a catalyst rejuvenating gas contacts the catalyst in the slurry to rejuvenate it and to form a rejuvenated catalyst slurry, and (iii) a means for returning catalyst to the slurry body. This design appears to be primarily for use as in-situ regeneration design. The “in-situ” regeneration offers the advantage of keeping the catalyst in the slurry matrix; however, it presents many challenges. Amongst other challenges in-situ regeneration, the H2 partial pressure in the process is limited due to the low solubility of H2 in the liquid phase. Typically, the H2 partial pressure exposed to the catalyst within the liquid phase is less than about 10% of that in the gas phase. In addition, the hydrogen used to regenerate may modify the H2:CO ratio in the reactor for some time. Further still, the temperature may be limited by the boiling point and/or cracking properties of the liquid slurry constituents. For these reasons, “in situ” regeneration has real limitations.
Further on it is known to regenerate a Fischer-Tropsch-Catalyst by a process for converting light hydrocarbons into heavier hydrocarbons (C5+) that includes regenerating a slurry Fischer-Tropsch catalyst in need of regeneration, the process comprising the steps of: preparing a synthesis gas using light hydrocarbons; converting the synthesis gas to Fischer-Tropsch products in a slurry Fischer-Tropsch reactor containing a slurry Fischer-Tropsch catalyst; removing Fischer-Tropsch products from the slurry Fischer-Tropsch reactor; regenerating the slurry Fischer-Tropsch catalyst that needs regeneration; and wherein the step of regenerating the slurry Fischer-Tropsch catalyst comprises the steps of: removing the catalyst from the slurry Fischer-Tropsch reactor; de-waxing and drying the catalyst sufficiently to produce a free-flowing catalyst powder that is fluidizable; fluidizing the catalyst powder; treating the catalyst powder with an oxygen treatment to remove hydrocarbons from the catalyst powder, reducing the catalyst powder with a reducing gas, re-slurring the catalyst powder to form a regenerated slurry catalyst; and returning the regenerated slurry catalyst to the slurry Fischer-Tropsch reactor (U.S. Pat. No. 6,989,403).